Injection compression moulding

ABSTRACT

A mould for injection compression moulding of an article includes a female mould part and a core for mounting in use on the platens of an injection moulding machine for movement towards and away from one another between an open and a closed position. A sealing ring surrounds the core for effecting a seal between the female mould part and the core. The core, the female mould part, and the sealing ring together define a closed mould cavity as the core approaches the closed position. The sealing ring has tapering sections, each tapering section having a tapering surface that seals against a tapering surface on the core only after the core has reached the closed position, leaving a venting clearance between the tapering surfaces of tapering sections of the sealing ring and the tapering surface of the core as the core approaches the closed position.

RELATED APPLICATION

This application is based upon and claims priority from prior BritishPatent Application No. 0400936.1, filed on Jan. 16, 2004 and PCT PatentApplication No. PCT/GB04/005422 filed Dec. 30, 2004 the entiredisclosures of which are hereby incorporated by reference in theirentirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to injection compression moulding and isconcerned in particular with injecting a plastics material into a mouldcavity.

2. Description of the Related Art

In the injection compression moulding technique to which the presentinvention relates, an accurately measured quantity of plastics materialis injected into a mould cavity before it has been fully closed. As theparts of the mould are brought together, the injected plastics materialis compressed and made to fill the cavity by the force applied to closethe mould rather than by the pressure applied to inject the plasticsmaterial into the mould. As a consequence, it is possible to achievemuch higher length to thickness ratios than achievable by conventionalinjection moulding, even when using lower cost plastics materials havinghigh viscosity. This enables the technique to be used in the manufactureof such items as cups and margarine tubs which have hitherto needed tobe manufactured by other methods, such as by vacuum or pressure formingof a heat softened sheet material. A further advantage is the greatlyreduced cooling times due to lower processing temperatures and improvedpacking allowing for faster heat loss through the cavity (conventionally70% of the heat loss is expected through the core due to shrinkage awayfrom the cavity wall).

Publication number WO/2002/058909, which is believed to represent theclosest prior art to the present invention, describes a mould formounting between the platens of an injection moulding machine forinjection compression moulding of a thin walled article. The mouldcomprises a female mould half mounted on the stationary platen of amoulding machine and a pressure plate which is mounted on the movingplaten and carries a core for closing the mould (there is no reason whythese two parts cannot be interchanged if desired). The core passesthrough, and is sealed relative to, a cylindrical bore in a rim closurering arranged between the female mould half and the pressure plate. Inuse, as the pressure plate is advanced towards the stationary mouldhalf, the rim closure ring is used to seal the mould cavity before thecore reaches its end position. Thus, when the plastics material isinjected into the mould cavity, it is fully sealed even though the corehas yet to be fully advanced into the mould to reduce its volume to itssmallest size.

It has been found in practice, however, that it is difficult to form arim closure ring with a cylindrical bore that effectively seals aroundthe core and yet allows the core to pass freely through it. Theclearance required to permit reliable and free movement of the corerelative to the rim closure ring does not permit creation of a perfectseal and results in an unacceptable witness line around the rim of themoulded article.

In conventional injection moulding, articles are moulded by first fullyclosing the mould. Next, a plastics material is injected into the mouldcavity to fill it completely. After the plastics material has beenallowed to cool sufficiently, the mould is opened, the moulded articleis ejected and the cycle is repeated. The above technique however placesa limitation on the length to thickness ratio of the moulded article.The minimum wall thickness that can be achieved varies with theviscosity of the plastics material and even to produce an article havinggreater wall thickness than is necessary for the structural integrity ofthe moulded article requires the use of more expensive low viscosityplastics materials.

By contrast, in injection compression moulding, at the end of theinjection of the plastics material, the core is not in the fully closedposition of the mould cavity. As the core is advanced towards the closedposition, the injected plastics material is forced to fill the mouldcavity. The plastics material is made to flow by the compression of themould cavity rather than by the injection pressure, and this has manyadvantages that are documented in the prior art.

Injection compression moulding does however present certain problemsthat are not encountered in conventional injection moulding. The firstof the problems is concerned with the alignment of the mould parts.Conventionally, conical mating surfaces are provided on the differentparts of the mould which centre them relative to one another when themould is fully closed. However, before the mould is fully closed, themould parts may not be fully aligned, that is to say they may not beconcentric or they may not be co-axial. The guiding that is achieved bythe tie bars, or other guiding systems used by machine manufacturers ofthe injection moulding machine, may not guarantee alignment to therequired accuracy, especially when it is noted that the main purpose ofusing injection compression moulding is to achieve very large flowlength to thickness ratios in articles such as cups, margarine tubs ordustbins.

A further problem in injection compression moulding is that ofcontaining the plastics material within the cavity as its volume isbeing reduced. In the prior art, this has been achieved by using a rimclosure ring to close off the cavity in the female mould part and by thecore passing through a cylindrical hole in the rim closure ring. This isnot a satisfactory solution because it is difficult to form a rimclosure ring with a cylindrical bore that effectively seals around thecore and yet allows the core to pass freely through it. The clearancerequired to permit reliable and free movement of the core relative tothe rim closure ring does not permit creation of a perfect seal andresults in an unacceptable witness line around the rim of the mouldedarticle.

SUMMARY OF THE INVENTION

With a view to mitigating the foregoing disadvantage, the presentinvention provides a mould for injection compression moulding of anarticle. The mould includes a female mould part, a core, and a sealingring. The core and the female mould part are movable relative to oneanother between an open and a fully closed position. The sealing ringsurrounds the core for effecting a seal between the female mould partand the core. The core, the female mould part, and the sealing ringtogether define the cavity when the core is in the fully closedposition. The sealing ring makes sealing contact with the female mouldpart prior to the core reaching the fully closed position. The sealingring has a tapering section that seals against a tapering surface on thecore only after the core has reached the fully closed position. There isa venting clearance between the tapering section of the sealing ring andthe core as the core is advanced towards the fully closed position tocompress plastics material injected into the mould cavity.

The present invention also provides an injection compression mouldingmachine that comprises a mould that includes a female mould part, acore, and a sealing ring. The core and the female mould part are movablerelative to one another between an open and a fully closed position. Thesealing ring surrounds the core for effecting a seal between the femalemould part and the core. The core, the female mould part, and thesealing ring together define the cavity when the core is in the fullyclosed position. The sealing ring makes sealing contact with the femalemould part prior to the core reaching the fully closed position. Thesealing ring has a tapering section that seals against a taperingsurface on the core only after the core has reached the fully closedposition. There is a venting clearance between the tapering section ofthe sealing ring and the core as the core is advanced towards the fullyclosed position to compress plastics material injected into the mouldcavity.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described further, by way of example, withreference to the accompanying drawings, in which:

FIG. 1 is a perspective view of a female mould part of a mould, lookinginto a cavity of the mould;

FIG. 2 is a perspective view of a core part of the mould, which fitsinto the female mould part of FIG. 1;

FIG. 3 is a perspective view of the female mould part of FIG. 1 and thecore part of FIG. 2, in a closed position of the mould;

FIG. 4 is a section through the core part of FIG. 2;

FIG. 5 shows part of the section of FIG. 4 drawn to an enlarged scale;and

FIG. 6 is a section through the mould when in the closed position.

DETAILED DESCRIPTION OF AN EXEMPLARY EMBODIMENT

The figures show a single core/cavity set for an injection compressionmould for making an article in the form of a drinking cup having agenerally flat base, a frustro-conical side wall and a lip in the formof an inverted “U” surrounding the mouth of the cup. It will beappreciated that the core cavity/set may be one of many in amulti-cavity mould and that the different sets can be arranged side byside and/or stacked back to back. The ensuing description will, however,for simplicity, refer to a single cavity mould.

Referring to FIGS. 1, 2 and 3, the mould comprises a female mould part10 and a core part 12 which fit into one another in the manner shown inFIG. 3 to leave between them a mould cavity having the desired shape ofthe drinking cup to be moulded.

In the illustrated embodiment of the invention, three flat guide fingers14 are firmly secured to the core part 12 to surround a central mouldcore 16 in accurately predetermined positions. Each of the guide fingers14 has two parallel-sided locating sections. One pair of locatingsections 14 a is provided near the base of each guide finger 14 and theother pair of locating sections 14 b is provided at its free end. Thewidth of the locating sections 14 a exceeds the width of the locatingsections 14 b. The portion of each guide finger in between the locatingsections 14 a and 14 b is shown as tapering gradually, but it may haveany shape provided that its width never exceeds the width of thelocating sections 14 a. Mating parallel-side locating sections 18 a and18 b are provided as part of U-shaped recesses defined by inserts 18similarly secures to the female mould part 10.

In use, the mould part that is not connected to the injection system,usually the core part 12, is mounted in such a manner as to allow it asmall degree of lost motion relative to its machine platen. Strongspring pressure is used to retain the mould part in position but, ifsufficient force is applied to it, the mould part will move laterally.

The first time the mould is fully closed, the fingers and inserts maynot mate perfectly with one another and this will apply a force to thecore part 12 to push it into alignment with the female mould part 10.When the mould is then closed fully, the parts of the mould are broughtinto perfect alignment with one another in the conventional manner.During subsequent operating cycles, the locating sections 14 a and 14 bwill interact with the surfaces of mating parallel-side locatingsections 18 a and 18 b before the mould is fully closed, and willeffect, in two different and axially separated planes, any minorrelative displacement of the mould parts that is necessary to assurecorrect alignment of the mould parts both in terms of concentricity andparallel alignment before the mould is fully closed.

Even though the guide fingers 14 and the inserts 18 ensure theconcentricity of the mould parts at both ends of the mould cavity, theydo so without the use of locating sections having an axial lengthmatching that of the mould cavity. Instead, no force is applied to alignthe mould parts until the mould is nearly fully closed. This isimportant as it avoids excessive wear to the locating sections.

One could consider using conical rods in place of the flat guide fingers14 but the latter are preferable because each finger is only called uponto effect a correction in one plane thereby simplifying the positioningof the locating sections on the mould parts.

In a multiple cavity mould, it is preferred to provide guide fingersaround each individual core/cavity set to allow for possible movement ofthe sets relative to one another. On smaller products, it may bepossible to provide guide fingers around two or four cavities if theyare tightly grouped together.

Though the illustrated embodiment of the invention uses three guidefingers 14 to align each mould, it is possible to use more, four beingpreferred.

A further problem that has to be overcome in injection compressionmoulding is that of containing the plastics material within the cavityas its volume is being reduced. In the prior art, this has been achievedby using a rim closure ring to close off the cavity in the female mouldpart and by the core passing through a cylindrical hole in the rimclosure ring. This is not a satisfactory solution because it isdifficult to form a rim closure ring with a cylindrical bore thateffectively seals around the core and yet allows the core to pass freelythrough it. The clearance required to permit reliable and free movementof the core relative to the rim closure ring does not permit creation ofa perfect seal and results in an unacceptable witness line around therim of the moulded article.

Referring now to FIGS. 4-6, a sealing ring 20 surrounds the core 16. Thesealing ring is held captive on the core part 12 and is capable of onlya small degree of movement relative to the core part in the direction ofthe axis of movement of the core part. Strong springs (or gas pressure)capable of withstanding the pressure within the mould act to hold thesealing ring 20 against the top surface of the female mould part 10 sothat no plastics material can escape laterally from the top of the mouldcavity.

Referring now to FIG. 5, a sealing surface between the sealing ring 20and the core 16 is not cylindrical, as in the prior art, but is formedof two contiguous tapering sections 22 and 24 of which the taperingsection 24 has a very small angle of taper, less than 5° and preferablyof the order of 1°, and the tapering section 22 has a larger angle oftaper. In both tapering sections 22 and 24, sealing contact between thesealing ring 20 and the core 16 does not occur before the mould cavityis fully closed.

In a typical operating cycle, the core 16 is first fully advanced intothe cavity in the female mould part 10 to exclude most of the air fromthe cavity. Next, an accurately measured quantity of plastics materialis injected into the mould cavity to form a biscuit at the base of themould cavity. During this time, the core 16 recoils slightly from thefemale mould part 10 either by the action of the injection pressure orby movement of the core cavity.

The sealing ring may or may not come into contact with the front of thefemale mould part 10 before the recoiling movement commences, dependingon the maximum stroke of the ring and the thickness of the biscuit thatis injected into the mould. If the sealing ring does contact the frontof the female mould part 10, then, depending on the stroke of the ringrelative to the core and the amplitude of the recoiling movement, it mayremain in sealing contact with the female mould part during the whole,or only the initial, part of the recoiling movement.

During the recoiling movement, the core 16 is maintained in alignmentwith the female mould part by the action of the guide finger 14. Theaxial movement of the core 16 relative to the sealing ring 20 opens agap between the two because of the tapering surfaces of the sealingtapering sections 22 and 24. However, because of the steepness of theangle of taper of the tapering section 24, the width of the gap that iscreated is only wide enough to act as a vent to allow gas to escape fromthe cavity.

When the core 16 is next advanced into the cavity of the female mouldpart 10, the biscuit of plastics material is compressed and is forced toflow up the side walls of the cavity towards the rim of the container.During this time, gas is expelled from the mould cavity first throughthe gap between the sealing ring and the cavity then through the gapbetween the sealing ring 20 and the core 16. As the core 16 reaches itsend position, the gap between it and the sealing ring 20 is closed fullyso as to prevent any egress of the plastics material from the mould.

Because of the accurate axial alignment which is achieved by using thetapered guide fingers 14 and the U-shaped sections of inserts 18, theplastics material flows at an even rate around the entire periphery ofthe cavity and reaches the end of the cavity at substantially the sametime. This reduces the distance that the sealing ring 20 needs to moverelative to the core 16.

It will be noted that the sealing ring not only closes the mould cavityefficiently to avoid any flashing but it does so without rubbing againstthe core 16. Furthermore, the sealing ring provides a vent thatdecreases in cross sectional area as the core 16 reaches the closedposition. Thus, at the commencement of the compression stroke when airneeds to be expelled from the cavity, the air can pass freely firstbetween the sealing ring 20 and the cavity of the mould 10, then betweenthe sealing ring 20 and the core 16. This avoids high temperatures beingreached in the vent and reduces air damping of the movement of the core16. By the time the gap is finally sealed off, all the air will havebeen evacuated from the mould cavity and the seal will prevent anyflashing of the plastics material.

The female mould part 10 and the core part 12 have been illustrated ingreater detail than is necessary for an understanding of the inventionbut the parts that have not been described in detail are generallyconventional and their function will be understood by the person skilledin the art without the need to detailed explanation. In particular,parts have been illustrated that are associated with such functions asthe injection of the plastics material into the cavity, the cooling ofthe moulding, the ejection of the mould article from the mould and themounting of the parts in the mould tool that is mounted to the platensof an injection moulding machine.

The material that is to be moulded is conventionally a moltenthermoplastic material. It should however be clear to the person skilledin the art that the invention will also find application in mouldingmolten metals, resins and thermosetting materials. Indeed, the materialcan be any material that is initially sufficiently fluid to be capableof being injected and that will subsequently harden, be it by cooling,heating or chemical curing.

In the present invention, the purpose of the tapering surface of thesealing ring that seals against the core 16 is to maintain a gap betweenthe core and the sealing ring until the mould is fully closed allowingfree movement of the sealing ring relative to the core. This is to becontrasted with tapering surfaces provided on sealing rings foralignment purpose, as exemplified by U.S. Pat. No. 6,500,376, where thesurfaces meet and leave no gap between them before the mould cavity isfully closed.

The term “sealing ring” is not, of course, to be construed in thepresent context to be restricted to a circular ring, as its outline willin each case be dictated by the outline of the article to be moulded.

Preferably, the angle of taper measured relative to the direction ofmovement of the core 16 is small, typically less than 5°, so that only asmall gap is present between the core and the sealing ring during thelast few millimeters of movement of the core. The width of the small gapthat remains as the core 16 approaches the closed position will notallow the injected material to penetrate into it but allows air toescape from the mould cavity.

Because air can escape from the cavity at any time before it is fullyclosed, it is possible to dispense entirely with the venting gaps thatare normally required when injection or injection compression mouldingan article. The ability to dispense with venting gaps has many importantadvantages. First, because gas has to escape through a vent that is toosmall to allow the injected material to flow through it, it is heated toa high temperature with the result that the venting gaps requireextensive maintenance and can reach a sufficiently high temperature toscorch the plastics material. Second, the back pressure resulting frompumping air through the venting gaps reduces the speed of movement ofthe injected material and the filling speed of the mould.

The sealing ring is preferably mounted on the core by a connection thatallows it a limited degree of movement relative to the core, and thesealing ring is urged by at least one of a spring and gas pressure in adirection to increase a size of the venting gap between taperingsurfaces.

It will also be appreciated that the mould as described needs to bemounted in a moulding machine that moves the core part 12 and the femalemould part 10 relative to one another at the appropriate rate whileapplying appropriate pressures. It has been found that thepressure/distance profile of a toggle operated moulding machine isideally suited to the injection compression moulding process but othermachines can be programmed to achieve a similar pressure/distanceprofile. When a machine is not capable of changing smoothly from a lowpressure large displacement mode to a high pressure small displacementmode, a further possibility would be to include a module between themould and the machine platens that is capable of delivering the desireddistance/pressure profile.

Although a specific embodiment of the present invention has beendisclosed, it will be understood by those having skill in the art thatchanges can be made to this specific embodiment without departing fromthe spirit and scope of the present invention. The scope of the presentinvention is not to be restricted, therefore, to the specificembodiment, and it is intended that the appended claims cover any andall such applications, modifications, and embodiments within the scopeof the present invention.

1. A mould for injection compression moulding of an article, the mouldcomprising: a female mould part; a core, the core and the female mouldpart being movable relative to one another between an open and a fullyclosed position; and a sealing ring surrounding the core for effecting aseal between the female mould part and the core, wherein the core, thefemale mould part, and the sealing ring together define a cavity whenthe core is in the fully closed position, such that the sealing ringmakes sealing contact with the female mould part prior to the corereaching the fully closed position and such that the sealing ring has atapering section that seals against a tapering surface on the core onlyafter the core has reached the fully closed position, the mouldincluding a venting clearance between the tapering section of thesealing ring and the core as the core is advanced towards the fullyclosed position to compress plastics material injected into the mouldcavity.
 2. The mould of claim 1, wherein the tapering section of thesealing ring has an angle of taper measured relative to a direction ofmovement of the core of less than 5°.
 3. The mould of claim 2, whereinthe angle of taper is substantially 1°.
 4. The mould of claim 1,including a venting gap to vent gas from the mould cavity, the ventinggap located between the tapering section of the sealing ring and thecore.
 5. The mould of claim 4, wherein the mould includes only a singleventing gap.
 6. The mould of claim 4, wherein the sealing ring ismounted on the core so as to be capable of a limited movement relativeto the core, the sealing ring being urged by at least one of a springand gas pressure in a direction to increase a size of the venting gap.7. The mould of claim 1, wherein at least one of the core and the femalemould part is adjustably mountable on a platen of an injection mouldingmachine to permit the core and the female mould part to be axiallyaligned with one another, wherein a plurality of guide fingers isprovided on one of the mould parts and distributed about an axis of thecore to be received in recesses of complementary shape in the othermould part as the mould parts approach the fully closed position, eachfinger having two parallel-sided locating sections that are spaced fromone another along a length of the finger, the locating section nearer afree end of the finger being narrower than the other and a length ofeach locating section being at least equal to a final part of a strokeof platens during which compression of injected plastics material takesplace.
 8. The mould of claim 7, wherein a part of each guide fingerlying between the two parallel-sided locating sections is continuouslytapering.
 9. The mould of claim 7, wherein each guide finger is flat andis secured to the core and the recesses are formed in a plurality offlat inserts releasably secured to the female mould part.
 10. Aninjection compression moulding machine, comprising: a mould, the mouldincluding, a female mould part, a core, the core and the female mouldpart being movable relative to one another between an open and a fullyclosed position, and a sealing ring surrounding the core for effecting aseal between the female mould part and the core, wherein the core, thefemale mould part, and the sealing ring together define a cavity whenthe core is in the fully closed position, such that the sealing ringmakes sealing contact with the female mould part prior to the corereaching the fully closed position and such that the sealing ring has atapering section that seals against a tapering surface on the core onlyafter the core has reached the fully closed position, the mouldincluding a venting clearance between the tapering section of thesealing ring and the core as the core is advanced towards the fullyclosed position to compress plastics material injected into the mouldcavity.
 11. The injection compression moulding machine of claim 10,wherein the tapering section of the sealing ring has an angle of tapermeasured relative to a direction of movement of the core of less than5°.
 12. The injection compression moulding machine of claim 11, whereinthe angle of taper is substantially 1°.
 13. The injection compressionmoulding machine of claim 10, including a venting gap to vent gas fromthe mould cavity, the venting gap located between the tapering sectionof the sealing ring and the core.
 14. The injection compression mouldingmachine of claim 13, wherein the mould includes only a single ventinggap.
 15. The injection compression moulding machine of claim 13, whereinthe sealing ring is mounted on the core so as to be capable of a limitedmovement relative to the core, the sealing ring being urged by at leastone of a spring and gas pressure in a direction to increase a size ofthe venting gap.
 16. The injection compression moulding machine of claim10, wherein at least one of the core and the female mould part isadjustably mountable on a platen of an injection moulding machine topermit the core and the female mould part to be axially aligned with oneanother, wherein a plurality of guide fingers is provided on one of themould parts and distributed about an axis of the core to be received inrecesses of complementary shape in the other mould part as the mouldparts approach the fully closed position, each finger having twoparallel-sided locating sections that are spaced from one another alonga length of the finger, the locating section nearer a free end of thefinger being narrower than the other and a length of each locatingsection being at least equal to a final part of a stroke of platensduring which compression of injected plastics material takes place. 17.The injection compression moulding machine of claim 16, wherein a partof each guide finger lying between the two parallel-sided locatingsections is continuously tapering.
 18. The injection compressionmoulding machine of claim 16, wherein each guide finger is flat and issecured to the core and the recesses are formed in a plurality of flatinserts releasably secured to the female mould part.